Ion treatment apparatus

ABSTRACT

A method and apparatus are disclosed for providing heat conduction between an article being treated in a vacuum and a support member by providing a gas under pressure of about 0.5 to 2.0 Torr between the article and the support member. The method and apparatus are described for use in a semiconductor wafer ion implantation system wherein the wafer is clamped to the support member which is cooled. A seal can be provided between the wafer and the support member adjacent the periphery of the article.

This application is a continuation of U.S. Ser No. 523,908, filed Aug.16, 1983, now abandoned; and a continuation of U.S. Ser. No. 185,075,filed Sept. 8, 1980, now abandoned; which was a division of U.S. Ser.No. 75,401, filed Sept. 14, 1979, and now U.S. Pat. No. 4,261,762,issued Apr. 14, 1981.

BACKGROUND OF THE INVENTION

In many applications where articles are treated within a vacuum chamberit is desirable to control the temperature of the article. One suchapplication is ion implantation of semiconductor wafers wherein a highenergy ion beam is directed onto a semiconductor wafer which alsoresults in heating of the wafer. Heating of the wafer in any ion implantprocess has a number of undesirable effects including damage to thephotoresist layer which can out gas and shrink thereby destroying thedesired precise pattern intended on the wafer by use of the photoresist.Early implantation systems relied upon heat removal from the siliconwafer by radiation effect only. Absence of gas molecules in the vacuumsystem such as typically 7×10⁻⁷ Torr virtually eliminates conductivepaths for heat flow. As beam powers in ion implantation systems haveincreased radiation cooling alone was no longer sufficient, and therehave been attempts to make intimate contact with the silicon wafer forincreased conduction. One method that has been attempted is the use of athermally conductive conformat (soft pliable material) pressedmechanically to the back of the silicon slice hopefully to establish asmany point contacts between the wafer and conformat for conduction to asupport member. Although significant temperature depression has beenrealized with the use of a conformat, problems of repeatability, thermalnon-uniformity and expensive maintenance have been experienced.

SUMMARY OF THE INVENTION

The object of the present invention is a method and apparatus which willprovide proper thermal conduction between an article and a supportmember in a vacuum environment and which will overcome the shortcomingsof the prior techniques.

Briefly stated, the invention to be described in greater detail below isdirected to method and apparatus for conducting heat between an articlebeing treated and a mounting member. In accordance with the presentinvention a gas under pressure is introduced between the article and themounting member and provides sufficient conductivity to allowtemperature control of the wafer.

Conductivity of a gas versus pressure is relatively flat fromapproximately 3,000 psi to 5/760 of an atmosphere at which levelconductivity decays rapidly. Experimentation has shown that gas betweena silicon wafer and a flat plate in the pressure range of about 0.5 to 2Torr has a very low diffusion rate into the void of the vacuum systemwhile at the same time providing sufficient thermal conductivity tomaintain the temperature of the wafer at appropriate levels.

In accordance with another aspect of the present invention the wafer isclamped against the support member which both limits the excess flow ofgas as well as produces a small path length for thermal conduction.

In accordance with another aspect of the present invention a seal can beprovided between the wafer and the support member adjacent the peripheryof the wafer thereby further limiting the escape of the gas into thevacuum chamber.

Other features and advantages of the present invention will become moreapparent upon a perusal of the following specification taken inconjunction with the accompanying drawings wherein similar characters ofreference refer to similar structural elements in each of the severalviews.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic block diagram illustrating one application of thepresent invention.

FIG. 2 is a schematic plan view of an ion implantation systemincorporating the present invention.

FIG. 3 is a schematic elevational sectional view of a portion of thestructure shown in FIG. 2 taken along line 22 in the direction of thearrows.

FIG. 4 is a schematic elevational view, partially broken away,illustrating a portion of the structure shown in FIG. 3.

FIG. 5 is a graph showing thermal conductance plotted versus gaspressure for nitrogen gas.

FIG. 6 is a graph of temperature plotted versus ion beam power showingfor the temperature of a silicon wafer in an ion implantation systemboth for an uncooled wafer and a wafer cooled in accordance with thepresent invention with nitrogen gas at two different pressure levels.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the present invention is applicable to provide thermalconductivity for controlling the temperature of an article in a vacuumfor numerous possible applications, it is especially applicable forcooling a semiconductor wafer in an ion implantation system. Accordinglythe invention will be described below with respect to such an ionimplantation system.

Referring now to the drawings particularly with reference to FIGS. 1 and2 there is schematically illustrated an ion implantation system whereinions from a source 11 connected to a high voltage power supply 12 aregenerated for projection through an accelerator column 13 along a beamline 14 to an end station 15 wherein the ions are directed against asemiconductor wafer. The source 11, column 13, beam line 14, and endstation 15 contained within a vacuum envelope 17 are maintained underhigh vacuum by vacuum pumping devices 16. The ion implantation system istypically operated at about the level 7×10⁻⁷ Torr when the ion beam isdirected against the wafer.

FIG. 2 better illustrates the elements of the ion implantation system.Ions from the source 11 are redirected by an analysing magnet 21 beforebeing directed through the accelerator column 13 and after which passthrough a triplet quadruple lens 22 and scanners 23. At the end station15 wafers 24 from an imput cassette 25 are directed to an inlet station26 through a vacuum lock 27 and into the high vacuum chamber 17 to thetreating station 28 where the wafer 24 is exposed to the ion beam. Fromthe treating station the wafer passes through a vacuum lock 29 to anoutput cassette 31 at the outlet station 32.

FIG. 3 schematically illustrates the structure and movement of the waferfrom the input cassette 25 to the output cassette 31. As shown in FIG. 3the wafer from cassette 25 passes through a first gate valve 33 to awafer stop 34 at which time the gate valve 33 is closed and the vacuumlock 27 reduced to an intermediate vacuum pressure. Then a second gatevalve 35 is opened and the wafer fed by gravity onto a target block orsupport plate or member 36 at a stop 40 at the treating station 28.Typically, the wafer is clamped to the target block 36 which is thentilted by a swing arm 37 for application of the appropriate ion dosage.The target block 36 is then swung down so that the wafer is releasedfrom the clamp and moves by gravity on through the third open gate valve38 to a stop 39 in vacuum lock 29. Gate valve 38 is then closed and afourth gate valve 41 opened whereby the wafer is fed by gravity to theoutput cassette 31.

FIG. 4 schematically illustrates the positioning and clamping of thewafer 24 on the target block 36 which may be cooled via a cooling systemsuch as freon circulated through internal passageways 36" from a coolantrecirculation system 42. The wafer 24 is clamped to the target block 36by a clamp 43 that is centrally apertured at 43' to pass the ion beamand that engages the wafer 24 adjacent its periphery.

Gas under pressure of about 0.5 to 2.0 Torr is fed through a channel 36'to the interface between the wafer 24 and the target block 36 andprovides the thermal conductivity for transferring heat from the waferto the cooled target block. A gas with a high thermal conductivity suchas, nitrogen, neon, helium or hydrogen (which are arranged in ascendingorder of conductivity at 360° K.) is directed from a source 44 through aregulator 45 and leak valve 46 to the channel 36'. It has been foundthat an opening at the end of the chanel 36' of approximatley 10 to 20thousanths inch diameter is sufficient to provide the appropriate gasfor maintaining a 3" wafer 24 at the desired temperature.

FIG. 5 shows a graph of thermal conductance plotted versus gas pressurefor nitrogen. It will be seen that the thermal conductivity remains highto approximately 5 Torr where it begins to fall off dramatically. Use ofgas in the range of 0.5 to 2.0 Torr provides the appropriate thermalconductivity for conducting heat away from the wafer. FIG. 6 shows agraph plotting temperature of a wafer against ion beam power and showsthe effectiveness of the use of nitrogen gas at 0.8 Torr and 1.5 Torr.

While it has not been found necessary for successful utilization of thepresent invention, a seal can be provided between the wafer 24 and thetarget block 36 adjacent the periphery wafer 24 by an "O" ring 47.

It will be appreciated by those skilled in the art that selection of theparticular operating gas and pressure will depend upon the efficiencyand nature of the particular system operation.

Additionally it will be appreciated by those skilled in the art thatthis invention can be utilized for temperature control of wafers inother treating processes. A typical applicable process would be plasmaetching of semiconductor wafers in a planar etching system, well knownin the art and ion beam milling and electron beam annealing.

Other modifications and alternative configurations can be utilized inaccordance with the present invention which is limited only by the scopeof the appended claims.

I claim:
 1. An apparatus for treating substantially flat wafer articleswith a focused ion beam comprising:a high vacuum chamber adapted to beevacuated to a pressure a substantial number of orders of magnitudebelow 1 torr, sufficient to enable formation and focusing of said ionbeam, a treating station within said high vacuum chamber, means forproducing, focusing and directing said ion beam onto wafer articles atthe treating station, a mounting member having a substantially, flat,temperature-controlled support surface on which each wafer is positionedat the treating station, and means for controlling the temperature ofthe wafer comprising an open clamping member for engageing one flatsurface of said wafer at its periphery to clamp the other flat surfaceof said wafer against said substantially flat support surface of saidmounting member, said focused beam passing through the opening of saidclamping member to said one surface of said wafer, means forintroducing, to the interface between the surface of the wafer and thesurface of the mounting member, gas under subatmospheric pressure ofsufficient value to enable the gas to have substantial heat conductance,said pressure being greater than about 1 torr, whereby effective heattransfer by conduction is obtained between said surfaces of said waferand mounting member due to the presence of said conductive gas in theshort paths that exist between the urged-together surfaces while leakageof said gas into said high vacuum chamber is substantially inhibited bysaid clamping of said surfaces against one another, thereby to preventsaid gas from influencing said treatment.
 2. The apparatus of claim 1wherein said means for introducing said gas to said interface betweensaid surfaces is adapted to maintain said gas at a pressure about sixorders of magnitude greater than the pressure in said high vacuumchamber.
 3. The apparatus of claim 1 including elastomeric seal meansfor sealing the wafer article to the mounting member adjacent theperiphery of the article.
 4. The apparatus of claim 1 including meansfor cooling the mounting member whereby the wafer being treated iscooled by heat conduction from the wafer via the gas to the cooledsupport surface against which it is clamped.
 5. Apparatus for treatingsemiconductor wafers with a focused ion beam comprising:a high vacuumchamber adapted to be evacuated to a pressure a substantial number oforders of magnitude below 1 torr sufficient to enable formation andfocusing of said ion beam, a treating station within said vacuum chamberincluding a mounting member for the wafer, said mounting memberincluding a substantially flat, temperature-controlled support surfaceengageable by a surface of said wafer, said support surface adapted toenable said wafer to slide face-to-face thereupon, gravity slide meansfor enabling the wafer to slide from a first position to the treatingstation and onto said flat surface of said mounting member, means at theperiphery of said wafer for clamping a flat surface of said waferagainst said support surface, means for producing, focusing anddirecting an ion beam to the treating station along an ion beam path,means for exposing the opposite surfaces of said wafer to said ion beamwhile said wafer is clamped against said support surface, means forintroducing, to the interface between the surfaces of said wafer andmounting member, gas under subatmospheric pressure of sufficient valueto enable the gas to have substantial heat conductance, said pressurebeing greater than about 1 torr, and means enabling the wafer to slideoff of said mounting member to a further position.
 6. The apparatus ofclaim 5 wherein said mounting member is tiltable from a wafer receivingposition via gravity feed, to a position where it intercepts the line ofsaid beam, said tiltable mounting member having a passage therethroughfor said gas and including means for cooling said mounting memberwhereby the wafer being treated is cooled by heat conduction from thewafer through the gas to said tiltable mounting member.